1. Field of the Invention
The present invention relates generally to planar circuits, more particularly to microstrip filters, and still more particularly to a microstrip resonator having a capacitor-loaded inductor wherein a capacitive element connected in parallel with an inductor is also the primary shunt capacitor of the resonator.
2. Description of the Related Art
Bandpass filters have wide applications in the today's communication systems. The escalating demand for communication channels dictates better use of frequency bandwidth. This demand results in increasingly more stringent requirements for RF filters used in the communication systems. Some applications require very narrow-band filters (as narrow as 0.05% bandwidth) with high signal throughput within the bandwidth. The filter shape must have sharp skirts so that a maximum amount of the available bandwidth may be utilized. Further, there is an increasing demand for small base stations in urban areas where channel density is high. In such applications, very small filter sizes are desirable.
One approach to the problem of obtaining filters with sharp skirts and high throughput has been to make filters using low-loss thin-film high-temperature superconductors (HTS). These filters are usually of microstrip design. The size of individual filter elements in HTS microstrip filters is limited not only by the requirements of the base station, but also by limitations in the available sizes of suitable substrates and deposition equipment.
FIGS. 1(a), 1(b), and 1(c) show a simple, pseudo-lumped element microstrip band pass filter and its equivalent circuit. The filter consists of a planar dielectric substrate 100 (FIG. 1(b)), on one side of which is a conductive ground plane 120. The planar circuitry 140 on the opposite side consists of a plurality of resonators, as shown in FIG. 1(a). Each resonator consists of two large end patches that approximate shunt capacitors at the resonator ends in the equivalent circuit, and a narrow transmission line in the middle that both approximates the middle resonator inductor and contributes small shunt capacitors at the resonator ends. The amount of coupling between the resonators is controlled by the gap size S, as shown in FIG. 1(a), which determines the series capacitance values in the equivalent circuit. For very narrow band filters with bandwidth well below 1%, the required gap size S can be quite large because the requirement of a very small series coupling capacitance. The large end patches also introduce significant stray couplings between non-adjacent parts of the circuit. The effect of stray couplings can significantly distort the filter response, which makes precise control of the couplings quite difficult. In addition, the size of the filter can be undesirably large due to the large gap size required.
Zhang, et al. U.S. patent application Ser. No. 08/706,974, titled “Frequency Transformation Apparatus and Method in Narrow-Band Filter Designs” and Zhang, et al., “Narrowband Lumped-Element Microstrip Filters Using Capacitively-Loaded Inductors”, IEEE Transactions on Microwave Theory and Techniques, vol. 43, No. 12, pp. 3030–3036 (1995) disclose using capacitively-loaded inductors to effectively scale down filter bandwidth, see FIG. 2 (example of a band-pass filter) and FIG. 4(a) (the equivalent circuit of the device shown in FIG. 2). With a gap size S designed for wider bandwidth, the same filter can achieve a much narrower bandwidth. It is done by replacing inductors in FIGS. 1(a), 1(b), and 1(c) with capacitively loaded inductors. That is, the filter shown in FIG. 2 can use a gap size S that is significantly smaller in than the one for the filter shown in FIG. 1 while achieving the same bandwidth. Thus, much better control of the couplings between the resonators can be obtained. However, because the series capacitor is in parallel with the inductor, many long, very narrow fingers are needed to achieve the required larger serial capacitor and at same time retain smaller shunt capacitors. Such a configuration imposes more stringent requirements on manufacturing tolerances.
The present invention is directed to improving the characteristics of the above-described filters.